Many reasons exist for dispensing liquid adhesives, such as hot melt adhesives, in the form of a thin filament or strand with a controlled pattern. Conventional patterns used in the past have been patterns involving a swirling effect of the filament by impinging the filament with a plurality of jets of air. This is generally known as controlled fiberization (CF) in the hot melt adhesive dispensing industry. Controlled fiberization techniques are especially useful for accurately covering a wider region of a substrate with adhesive dispensed as single filaments or as multiple side-by-side filaments from nozzle passages having small diameters, such as on the order of 0.010 inch to 0.060 inch. The width of the adhesive pattern placed on the substrate can be widened to many times the width of the adhesive filament itself.
Controlled fiberization techniques are often used to provide better control over adhesive placement. This is especially useful along the edges of a substrate and on very narrow substrates, for example, such as on strands of material (e.g., LYCRA®) used in the leg bands of diapers. Other adhesive filament dispensing techniques and apparatus have been used for producing an oscillating pattern of adhesive on a substrate or, in other words, a stitching pattern in which the adhesive moves back-and-forth generally in a zig-zag form on the substrate. Typically, these dispensers or applicators have a series of liquid and air orifices arranged on the same plane.
Conventional swirl nozzles or die tips typically have a central adhesive discharge passage surrounded by a plurality of air passages. The adhesive discharge passage is centrally located on a protrusion that is symmetrical in a full circle or radially about the adhesive discharge passage. A common configuration for the protrusion is conical or frustoconical with the adhesive discharge passage exiting at the apex. The air passages are typically disposed at the base of the protrusion. The air passages are arranged in a radially symmetric pattern about the central adhesive discharge passage, as in the protrusion itself. The air passages are directed in a generally tangential manner relative to the adhesive discharge passage and are all angled in a clockwise or counterclockwise direction around the central adhesive discharge passage.
Conventional meltblown adhesive dispensing apparatus typically comprise a die tip having multiple adhesive or liquid discharge passages disposed along an apex of a wedge-shaped member and air passages of any shape disposed along the base of the wedge-shaped member. The wedge-shaped member is not a radially symmetric element. Rather, it is typically elongated in length relative to width. The air is directed from the air discharge passages generally along the side surfaces of the wedge-shaped member toward the apex, and the air impacts the adhesive or other liquid material as it discharges from the liquid discharge passages to draw down and attenuate the filaments. The filaments are discharged in a generally random manner.
Various types of nozzles or die tips, such as those of the type described above, have been used to dispense adhesive filaments onto one or more elastic strands. Each strand is typically aligned and directed by a guide proximate the corresponding adhesive discharge passage. The strands tend to acquire airborne particulates present in the environment surrounding the liquid adhesive dispensing apparatus. These airborne particulates consist of dust and other contaminants that primarily originate from the processing operations performed by the production line. In addition, the strands may be intentionally coated with particulates, such as talc, to facilitate movement through the guide.
As each strand interacts with the corresponding guide, the particulates, regardless of origin, may be wiped off and accumulate or agglomerate into larger masses. The agglomerated masses of particulates may dislodge from the guide and incorporate into the dispensed adhesive filament. For example, the agglomerated mass may be dislodged by a knot is formed between the trailing end of a first length of strand material and the leading edge of a second length of strand material joined to provide a continuous strand. Alternatively, the agglomerated mass may remain resident in the guide and increase in dimensions to such an extent that the strand itself is displaced or removed from the guide. In multi-strand dispensing operations, an adjacent guide may capture the displaced strand, which disrupts the application of adhesive to the strands and ultimately produces defective product because the strands are adhesively bonded to a substrate with improper positioning. The reduction in product quality may be significant and may increase the manufacturing cost.
Another difficulty associated with dispensing adhesive onto a guided, moving strand occurs during periods in which the production line is idled, such as for line maintenance. The strand or strands may be fixed in position and in contact with heated surfaces of the adhesive nozzle or die tip. Heat transferred from the nozzle or die tip to each strand may result in strand breakage because of temperature effects. As a result, the downtime of the production line may be increased for reconnection of the strand break or substitution of an unbroken strand.
Yet another difficulty associated with dispensing adhesive onto a guided, moving strand arises from contact between the strand and the adhesive nozzle or die tip. Specifically, the strand wears the metal surfaces of the nozzle or die tip and the metal surfaces of the guide or guides due to frictional wear. Eventually, the wear may necessitate replacement of the nozzle, die tip or guide. Moreover, the contact between the strand and these metal surfaces causes drag on the strand, which may reduce the predictability of adhesive application or may result in broken strands.
What is needed, therefore, is a liquid dispensing module for dispensing a liquid filament onto a substrate in which the difficulties associated with strand guiding are reduced or eliminated.